Method of making picture tube fluorescent screen

ABSTRACT

There are disclosed a composition for making a picture tube fluorescent screen characterized by essentially consisting of a light-sensitive resin at least having recurring units (1) and (2) represented by the formulas: ##STR1## wherein R and X -  have the same meanings as defined in the specification, and CaS-based phosphor particles surface-coated with SiO 2  which are dispersed in an aqueous medium, and a method of making a picture tube fluorescent screen using the same. 
     According to this invention, it is possible to make a picture tube exhibiting an excellent light emitting efficiency and a contrast by a simple process.

This invention relates to a composition for use in making a fluorescentscreen of a color picture tube and a method of making such a fluorescentscreen by using the composition, and more particularly, to a compositionand a fluorescent screen making method based on a CaS phosphor.

In general, fluorescent screens of shadow mask type color picture tubesare made as described below. Namely, an aqueous light-sensitive phosphorsuspension (hereinafter referred to as slurry) containing a polyvinylalcohol (PVA), ammonium dichromate (ADC) and phosphor particlesdispersed in an aqueous medium is applied to the inner surface of a faceplate panel of a picture tube, and the coat thus formed is exposed toultraviolet light through a shadow mask having a desired pattern toinsolublize the exposed section of the coat. The unexposed section ofthe coat is then washed away with water to obtain a dot-like orstripe-like phosphor pattern. The operation described above is repeatedfor each of green, blue and red phosphors to obtain a fluorescent screenof the color picture tube.

Materials for use in making fluorescent screens of picture tubes shouldexhibit a good light emitting efficiency, and a need exists for suchexcellent materials. Thus, it has been proposed to use a yttrium-basedphosphor exhibiting a high luminance. However, this material isexpensive. On the other hand, it has been studied to use CaS in makingfluorescent screens of picture tubes because CaS is cheap and exhibits ahigh luminance. However, when the CaS-based phosphor is added to theslurry of the conventional PVA-ADC light-sensitive material, the slurrygels about 10 minutes after the addition and cannot be applied to aninner surface of a face plate panel any more. Thus, it is practicallyimpossible to use the CaS-based phosphor in picture tubes. Morerecently, in order to eliminate this problem, it has been proposed toapply a material exhibiting moderate adhering and bonding properties tothe inner surface of the face plate panel and then apply the phosphor inthe dry particle form to the inner surface of the face plate panel tocause the phosphor to adhere thereto (see Japanese Provisional PatentPublication No. 46642/1981). However, according to this method, it needsextremely long period of time in the exposure to form a desired pattern,and it is difficult to make a sufficient amount of the phosphor toadhere to the inner surface of the face plate panel. Therefore, in spiteof using the CaS-based phosphor having high luminance, it does not formthe phosphor having sufficient high luminance and it is not suitable forpractical purposes.

The object of the present invention is to eliminate the problems of theprior art and provide a composition for making a fluorescent screen,which is based on a CaS phosphor exhibiting a high luminance and whichexhibits a stable, sufficient sensitivity and contrast and yet shows nogelation, and a method of making a fluorescent screen by using such acomposition.

The present invention relates to a composition (slurry) for making apicture tube fluorescent screen, essentially consisting of alight-sensitive resin at least having recurring units (1) and (2)represented by the following general formulas: ##STR2## wherein Rrepresents a hydrogen atom, an alkyl group or a lower-hydroxylalkylgroup; and X⁻ represents an anion of a strong acid, and CaS-basedphosphor particles which are surface-coated with SiO₂, said resin andsaid phosphor particles being dispersed in an aqueous medium. Accordingto the surface-coating of the particles, the stability of the phosphorslurry makes great improvement. The present invention also relates to amethod of making a picture tube fluorescent screen by using thecomposition.

FIG. 1 is a graph showing the effects of the present invention.

The light-sensitive resin used in the present invention contains therecurring units represented by the general formulas (1) and (2) as theessential constituents. R in the general formula (1) may be selectedfrom a hydrogen atom, an alkyl group or a lower-hydroxyalkyl group, andis preferably a hydrogen atom, or an alkyl or hydroxyalkyl group havingone to three carbon atoms. X⁻ in the general formula (1) is selectedfrom a halogen ion, a sulfate ion, a phosphate ion or ap-toluenesulfonate ion.

In the present invention, the light-sensitive resin having theproportion of the recurring unit (1) in the range between 0.1 to 10molecular percent is preferably used. If the proportion of the recurringunit (1) in the light-sensitive resin is above 10 mol %, the solubilityof the resin in water deteriorates, affecting the formation of uniformslurry. Further, in the firing process conducted after the development,a residue of the light-sensitive resin remains in the light-sensitivefilm, adversely affecting the characteristics of the resltingfluorescent screen. Proportions of the recurring unit (1) below 0.1 mol% result in a resin which exhibits insufficient sensitivity to light andwhich is not suitable for practical purposes. In the present invention,a light-sensitive resin having a degree of polymerization in the rangebetween 400 and 3000 is preferably used, and more preferably in therange between 800 and 2600.

In addition to the recurring units represented by the above-describedchemical formulas, a copolymerizable vinyl monomer constituent may becontained in the light-sensitive resin in order to improve thecharacteristics such as solubility in water and heat decomposingproperty. The vinyl monomer may, for example, be vinyl acetate, vinylpropionate, olefin, styrene, acrylic acid, methacrylic acid, acrylate,methacrylate or a derivative of one of these compounds.

The light-sensitive resin used in the present invention can be preparedby reacting a polyvinyl alcohol, partially saponified polyvinyl acetateor a copolymer of these compounds with a styryl pyridinium salt havingformyl group, as described in Japanese Patent Publication No. 5761/1981.

In the present invention, it is preferable that the light-sensitiveresin be used in a proportion between 1.5 and 4.0% by weight based onthe whole slurry. If the proportion of the light-sensitive resin isabove this range, the viscosity of the slurry rises to make it difficultto apply the slurry. If the proportion of the light-sensitive resin isbelow this range, the sensitivity of the slurry to light deteriorates,and the exposure pattern otained after the development becomesindistinct.

The CaS-based phosphor used in the present invention comprises CaSactivated with cerium, europium, silver or the like, and they arewell-known materials. On the surface of the phosphor is formed a coat ofSiO₂ in an amount between 0.001 and 4.0% by weight based on thephosphor, preferably in an amount between 0.002 and 1% by weight. It ispreferable that the thickness of the SiO₂ coat be in the range between0.5 to 70 mμ, more preferably between 0.01 to 50 mμ, particularlypreferably between 0.02 to 14 mμ, and the continuous coat be formedalmost all the surface of the phosphor. Such a coat can be formed byusing the technique as disclosed, for example, in Japanese ProvisionalPatent Publication No. 65286/1980.

Namely, a SiO₂ coat can be formed on the surface of the phosphor bydissolving SiO₂ in an alkaline solution containing a compound such ascholine, tetramethylammonium hydroxide or a similar compound thereofrepresented by the general formula [N⁺.R₄ ]OH⁻, in which R denotes analkyl group which may be the same or different, respectively. Then,mixing the thus formed SiO₂ solution, the phosphor and pure water witheach other in an appropriate ratio, followed by evaporation to dryness.

If the amount of the SiO₂ coat is less than 0.001% by weight based onthe phosphor, the characteristics of the slurry tend to deterioraterelatively in a short time. If the amount is over 4.0% by weight, theluminance of the phosphor deteriorates greatly.

In the present invention, it is preferable that the size of the phosphorparticles is in the range between 4 and 15μ, and the phosphor particlesbe contained in a proportion between 10 and 40% by weight based on thewhole slurry. If the proportion of the phosphor particles is above thisrange, the particles settle out in the slurry. If the proportion isbelow this range, the amount of the phosphor applied to the panel faceplate deteriorates.

The slurry in accordance with the present invention is obtained bydispersing the above-mentioned light-sensitive resin and the CaS-basedphosphor particles in a predetermined ratio in an aqueous medium. Theaqueous medium used in the present invention may be water or a mixtureof water and a polar organic solvent.

By adding an organic solvent, it is possible to improve the solubilityof the light-sensitive resin. The organic solvent used for this purposemay be an alcohol, dimethylformamide, formamide, or dimethylsulfoxide.If the obtained slurry is unstable and tends to form sediments, asurface active agent may be added to the slurry to stabilize it. It isalso possible to add known additives such as thickner and levellingagent. For easiness of the applying work, it is preferable that theviscosity of the slurry of the present invention be in the range between15 and 30 cps (at 21° C.).

A method of forming the fluorescent screen in accordance with thepresent invention will now be described below.

To form the fluorescent screen, a slurry adjusted the above-mentionedcomposition is prepared, applied to the inner surface of the face platepanel of a picture tube, and dried. Thereafter, the inner surface of theface plate panel is exposed to ultraviolet light in a desired pattern byuse of a shadow mask in order to make the light-sensitive resin in theresulting coat insoluble in water. For the exposure to ultravioletlight, it is possible to use any light source capable of emitting lightcontaining the ultraviolet component having a wavelength in the vicinityof 340 nm, for example, a high-pressure mercury vapor lamp. In theexposure, it is sufficient that the illuminance is in the range between0.5 and 2.0 mW/cm², and the exposure time is in the range between 2 and60 seconds. The coat thus exposed pattern-wise is then developed toremove the unexposed light-sensitive composition. The development iseffected by washing the coat with water or warm water.

Thereafter, the fluorescent screen thus formed is subjected to metalbacking treatment by use of aluminum or the like when necessary. By thisprocess, it is possible to compensate for a shortage in the amount oflight emitted from the fluorescent screen. Metal backing treatment isconducted by depositing a metal such as aluminum onto the surface of thefluorescent screen. It is preferable that, before conducting metalbacking treatment, an organic film be formed on the surface of thefluorescent screen by lacquering. The picture tube is then fired toadhere the phosphor onto the face plate panel. By the firing process,constituents other than the phosphor are removed by evaporation orthermal decomposition. The firing temperature is preferably between 350°and 450° C., and the firing time is preferably between 30 and 90minutes. If the firing temperature is lower than the abovementionedrange or if the firing time is shorter than the range described above,the constituents of the resin and the like are not removed completelybut remain in the fluorescent screen, adversely affecting the lightemitting performance.

By the process described above, a film of the CaS-based phosphor can beformed on the face plate panel of the picture tube. In order to make acolor picture tube, it is necessary to form phosphor layers of the othertwo colors before or after the formation of the CaS-based phosphorlayer. These phosphor layers may be formed in any sequence before themetal backing process. As the light-sensitive materials to be used forthe formation of the phosphor, it is possible to use the light-sensitiveresins in accordance with the present invention or to use theconventional PVA-ADC based materials.

In accordance with the present invention, it is possible to form thehigh-luminance CaS phosphor in a stable aqueous slurry uniformly on theface plate panel in a sufficient amount. Therefore, it is possible tomake a picture tube exhibiting an excellent light emitting efficiencyand a contrast by a simple process. In addition, since it is unnecessaryto use a detrimental chemical such as ADC, the present invention isadvantageous also for controlling polutions.

The present invention will hereinbelow be described in further detail bythe following examples.

EXAMPLE 1

Particles of green light-emitting phosphor CaS:Ce were washed clean withethanol, filtered and dried at a temperature of about 80° C. On theother hand, an SiO₂ solution was prepared by dissolving 10% of SiO₂ in a10% aqueous choline solution. Then, 3 parts by weight of the SiO₂solution thus formed were added to 200 parts by weight of deionizedwater, and the resulting mixture was stirred for about 10 minutes. Tothe solution thus obtained were then added 100 parts by weight of theabove-described CaS:Ce phosphor. The suspension thus prepared wasstirred for about 15 minutes, subjected to suction filtration, andallowed to stand for about 1 hour. The solid contents thus obtained weredried for 10 hours at a temperature between 80° and 120° C., and thensifted through a 500-mesh sieve to obtain CaS:Ce phosphor coated with0.015% by weight of SiO₂.

Thereafter, the slurry having the composition described below andcontaining the thus obtained CaS:Ce phosphor was applied on a face platepanel and dried. The green phosphor pattern forming positions were thenexposed to an illuminance of 1.5 mW/cm² for 5 seconds in an exposurestand provided with a super-high-pressure mercury vapor lamp as thelight source, followed by development with warm water to form a CaSphosphor layer. Thereafter, layers of a blue phosphor and a red phosphorwere formed in the same way by use of the conventional ADC-PVA basedresist. On the phosphor layers thus obtained was formed an organic filmby the so-called lacquering method, and the an aluminum metal back wasformed, followed by firing at 430° C. for 30 minutes to make a colorpicture tube.

The CaS phosphor layer thus formed exhibites a luminance 10% improvedover the conventional method since it was possible to make a desiredamount of the phosphor adhere to the face plate panel sufficiently.

    ______________________________________                                        CaS:Ce green phosphor  30       parts                                         Polyvinyl-α-methyl(γ-formylstyryl)                                                       3        parts                                         pyridinium-methosulfate                                                       A surface active agent 0.1      part                                          Pure water             about 32 parts                                         ______________________________________                                    

The conventional method described above was conducted as describedbelow.

Namely, a sensitive sticky mixture having the composition describedbelow was applied to a panel and dried. Then, the green phosphor patternforming positions were exposed to an illuminance of 1.5 mW/cm² for 180seconds in an exposure stand provided with a superhigh-pressure mercuryvapor lamp as the light source. CaS:Ce green phosphor particles werescattered onto the inner surface of a panel and made to adhere theretoin a required pattern. The surplus phosphor particles were then removedby an air spray. Thereafter, layers of a blue phosphor and a redphosphor were formed, respectively, in the same way by use of theconventional ADC-PVA resist.

The subsequent procedure was the same as in the method employed in thepresent invention. In this way, a color picture tube was made.

    ______________________________________                                        Dimethylaminobenzenediazonium                                                                          3% by weight                                         chloride-zinc chloride                                                        Propylene glycol alginate                                                                            0.6% by weight                                         Ethylene glycol        0.2% by weight                                         Water                  balance                                                ______________________________________                                    

In the conventional method, when the exposure time is to be varied about100 seconds, there was not formed the pattern at all.

EXAMPLE 2

Two kinds of slurries were prepared by mixing, in each case, 30 parts ofthe SiO₂ -coated CaS:Ce phosphor, which was prepared by the methoddescribed in Example 1, or 30 parts of the CaS:Ce phosphor, which wasnot coated with SiO₂, with 3 parts ofpolyvinyl-α-methyl(γ-formylstyryl)pyridinium-methosulfate, 0.1 part of asurface active agent and 32 parts of pure water. Further, theconventional slurry was prepared by mixing an ADC-PVA based phosphorwith CaS based phosphor.

The changes in the relative sensitivity of these three slurries withtime was determined, and the results were evaluated with the sensitivityof the slurries immediately after the preparation taken as 100. Theresults were shown in FIG. 1. As shown in FIG. 1, the sensitivity of theconventional phosphor with CaS phosphor dropped 15 minutes after thepreparation, and dropped to about half an hour. On the contrary, thephosphor in accordance with the present invention showed no significantreduction in sensitivity, particularly, the phosphor coated with SiO₂showed excellent stability in sensitivity even after 60 hours.

EXAMPLE 3

First, the green phosphor and the blue phosphor were sequentially formedin a predetermined pattern by use of an ADC-PVA based resist in the sameway as described in Example 3. Then, a CaS:Ce, Eu slurry having thecomposition described below and containing a CaS:Ce, Eu red lightemitting phosphor coated with SiO₂ in the same way as described inExample 1 was applied to the face plate panel and dried. The redphosphor pattern forming positions were then exposed to an illuminanceof 1.5 mW/cm² for 10 seconds in an exposure stand provided with asuper-high-pressure mercury vapor lamp as the light source, followed bydevelopment with warm water to form a CaS phosphor layer. Thereafter, acolor picture tube was made in the same way as described in Example 1.

The CaS phosphor layer thus formed exhibited a luminance 5% improvedover the conventional method described in Example 1 since it waspossible to make a desired amount of the phosphor adhere to the faceplate panel sufficiently.

    ______________________________________                                        CaS:Ce, Eu red phosphor                                                                              30       parts                                         Polyvinyl-α-methyl(γ-formylstyryl)                                                       3        parts                                         pyridinium-methosulfate                                                       A surface active agent 0.1      part                                          Pure water             about 32 parts                                         ______________________________________                                    

We claim:
 1. A method of making a picture tube fluorescent screen,comprising the steps of applying, to the inner surface of a face platepanel of a picture tube, a composition essentially consisting of alight-sensitive resin at least having recurring units (1) and (2)represented by the following general formulas: ##STR3## in which Rdesignates a hydrogen atom, an alkyl group or a lower hydroxyalkylgroup, and X⁻ designates an anion of a strong acid, and CaS-basedphosphor particles surface-coated with SiO₂, said light-sensitive resinand said CaS-based phosphor particles being dispersed in an aqueousmedium; exposing the coat thus formed to light in a required pattern byuse of a mask; developing and then baking the coat to form a fluorescentscreen in a required pattern.
 2. A method of making a picture tubefluorescent screen according to claim 1, wherein X⁻ is a halogen ion, asulfate ion, a phosphate ion or a p-toluenesulfonate ion.
 3. A method ofmaking a picture tube fluorescent screen according to claim 1, whereinlight-sensitive resin contains 0.1 to 10 mol % of the recurring unitrepresented by the general formula (1).
 4. A method of making a picturetube fluorescent screen according to claim 1, wherein SiO₂ coated onsaid CaS-based phosphor forms a continuous coat on almost all thesurface of the phosphor, and the required coat weight is in the rangebetween 0.001% and 4.0% based on the phosphor.